U.S. patent application number 12/657095 was filed with the patent office on 2010-09-30 for method of cleaning semiconductor wafers.
This patent application is currently assigned to Rohm and Haas Electronic Materials LLC. Invention is credited to Robert K. Barr, Raymond Chan, Matthew L. Moynihan.
Application Number | 20100248494 12/657095 |
Document ID | / |
Family ID | 42101815 |
Filed Date | 2010-09-30 |
United States Patent
Application |
20100248494 |
Kind Code |
A1 |
Barr; Robert K. ; et
al. |
September 30, 2010 |
Method of cleaning semiconductor wafers
Abstract
A method of cleaning semiconductor wafers using an acid cleaner
followed by an alkaline cleaner to clean contaminants from the
materials is provided. The acid cleaner removes substantially all
of the metal contaminants while the alkaline cleaner removes
substantially all of the non-metal contaminants, such as organics
and particulate material.
Inventors: |
Barr; Robert K.;
(Shrewsbury, MA) ; Chan; Raymond; (Westbororugh,
MA) ; Moynihan; Matthew L.; (Sterling, MA) |
Correspondence
Address: |
John J. Piskorski;Rohm and Haas Electronic Materials LLC
455 Forest Street
Marlborough
MA
01752
US
|
Assignee: |
Rohm and Haas Electronic Materials
LLC
Marlborough
MA
|
Family ID: |
42101815 |
Appl. No.: |
12/657095 |
Filed: |
January 13, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61205020 |
Jan 14, 2009 |
|
|
|
Current U.S.
Class: |
438/745 ;
257/E21.485 |
Current CPC
Class: |
H01L 21/02052
20130101 |
Class at
Publication: |
438/745 ;
257/E21.485 |
International
Class: |
H01L 21/465 20060101
H01L021/465 |
Claims
1. A method comprising, in sequence, the following: a) applying an
aqueous acid composition comprising one or more acids chosen from
polycarboxylic acids and inorganic acids to a semiconductor wafer;
and b) applying an aqueous alkaline composition comprising one or
more alkaline compounds to the semiconductor wafer to remove metals
and residue from the material.
2. The method of claim 1, further comprising a step of applying a
second acid composition comprising one or more acids chosen from
polycarboxylic acids and inorganic acids to the semiconductor wafer
subsequent to step b.
3. The method of claim 1, wherein the aqueous acid composition and
the aqueous alkaline composition further comprise one or more
surfactants.
4. The method of claim 3, wherein the one or more surfactants are
nonionic.
5. The method of claim 4, wherein the nonionic surfactant is chosen
from a polyol and a gemini surfactant.
6. The composition of claim 5, wherein the polyol is chosen from a
glycoside and a polyglycoside.
7. The composition of claim 1, wherein the polycarboxylic acid is
chosen from a dicarboxylic acid, a tricarboxylic acid and an
oxycarboxylic acid.
8. The composition of claim 1 wherein the alkaline compounds are
chosen from sodium carbonate, potassium carbonate, sodium
bicarbonate, sodium hydroxide, potassium hydroxide and tetra-methyl
ammonium hydroxide.
Description
[0001] The present invention is directed to a method of cleaning
semiconductor wafers of contaminants. More specifically, the
present invention is directed to a method of cleaning semiconductor
wafers of contaminants using an acid cleaner followed by an
alkaline cleaner.
[0002] Conventionally, semiconductor wafers may be fabricated by
the following steps: [0003] (1) a semiconductor ingot is sliced by
an inner diameter saw to obtain a wafer; [0004] (2) the wafer is
washed with water to remove contaminants; and [0005] (3) the wafer
is then cleaned to remove impurities including heavy metal and
particles and then dried.
[0006] When the semiconductor wafer is used in the manufacture of
solar cells, a surface of the wafer is roughened or textured to
reduce light reflectance. A solar cell is a device that converts
light energy incident on its surface such as sunlight into electric
energy. Various approaches have been attempted to increase the
absorbance of energy. One of such approaches is to reduce
reflectance of light incident on the surface of the solar cell.
Reducing light reflectance from the surface allows the conversion
efficiency into electric energy to be improved. Typically texturing
is done by etching a surface of the semiconductor wafer in a
predetermined condition with an alkaline aqueous solution such as
sodium hydroxide.
[0007] Prior to texturing, the semiconductor ingot is sliced to a
desired size and shape using an inner diameter saw as mentioned
above. During the cutting process metal from the saw such as iron,
copper, lead and zinc contaminate the surface of the sliced
semiconductor. In addition, slurries which are applied to the saw
during the cutting process also contaminate the semiconductor. By
cutting using a slurry that is either a mixture of free abrasive
particles, such as silicon carbide, and an organic material or oil
base such as a mineral oil, or a mixture of free abrasive particles
and an aqueous solution base such as polyethylene glycol, the
effects of improving cuttability, reducing residual processing
distortion, suppressing process stress and suppressing cutting heat
can be enhanced. Further, by the addition of alkaline hydroxides to
such slurries process stress (residual distortion) produced by
cutting is eliminated thus to cut low-distortion wafers. Such a
method is disclosed in U.S. Pat. No. 6,568,384. Such abrasive
particles and organic materials also contaminant the semiconductor
wafer and may compromise the texturing process thus to reduce the
desired absorbance of the final solar cell article. Accordingly,
removal of such surface contaminants is important prior to
texturing.
[0008] Conventional methods of cleaning the semiconductor wafer
prior to texturing, such as with water and alkaline cleaners, have
not been found to be satisfactory to many workers in the
semiconductor industry. Metals such as iron and copper are
difficult to remove, in particular iron. If iron is not removed
from the wafer, iron oxide, as indicated by black spots, forms on
the wafer compromising the optimum absorbance of the surface of the
wafer. Some alkaline cleaners have been found to remove organic
materials well but not metals. Further, in addition to cleaning
organics from the wafer, the alkaline cleaners have an etching
action, i.e., partial texturing. Partial texturing occurs when
hydroxide reacts with silicon to form soluble silicates that
dissolve into the aqueous solution. Ideally etching of silicon is
minimized during cleaning (zero etching is desired). This partial
texturing is noticeable as white spots. This is particularly common
when the semiconductor is a monocrystalline wafer. Accordingly,
there is a need in the semiconductor industry for an improved
method of removing contaminants from a semiconductor wafer prior to
texturing to improve the overall performance and appearance of the
semiconductor wafer used in solar cells; and minimize contamination
of the wafer with conducting metals which may reduce the efficiency
of the solar cells.
[0009] In one aspect a method includes, in sequence, the following:
[0010] a) applying a first composition including one or more acids
chosen from polycarboxylic acids and inorganic acids to a
semiconductor wafer; and [0011] b) applying a second composition
including one or more alkaline compounds to the semiconductor wafer
to remove metals, organics and abrasives from the wafer.
[0012] In another aspect a method includes, in sequence, the
following: [0013] a) applying a first composition including one or
more acids chosen from polycarboxylic acids and inorganic acids to
a semiconductor wafer; [0014] b) applying a second composition
including one or more alkaline compounds to the semiconductor
wafer; and [0015] c) applying a second acid composition including
one or more acids chosen from polycarboxylic acids and inorganic
acids to the semiconductor wafer to remove metals, organics and
abrasives from the wafer. [0016] The method is used to remove
contaminants on a semiconductor wafer after it has been cut from an
ingot. The saws used in the cutting process contaminate the surface
of the semiconductor with metals such as iron, copper, lead and
zinc. The slurries used during the cutting process contaminate the
surface with organics and abrasive particles. Such contaminants are
undesirable since they may compromise appearance and efficiency of
the semiconductor. Minimizing metal contamination is desired when
the semiconductor wafer is used in the manufacture of a solar cell.
Such contamination increases the probability that metal
contaminants may be incorporated into the silicon matrix resulting
in poor conductivity and an over all drop in solar cell efficiency.
In addition to cleaning metals, organics and abrasive particles
from semiconductor wafers, the method is compatible with ultrasonic
cleaning processes, low foaming, environmentally friendly, easy to
rinse from semiconductor wafers and flexible to customize to
customer processes.
[0017] As used throughout this specification, the following
abbreviations have the following meaning, unless the context
indicates otherwise: .degree. C.=degrees Centigrade; gm=gram;
L=liter; mL=milliliter; cm=centimeters; ppm=parts per million;
ppb=parts per billion; wt %=weight percent. All numerical ranges
are inclusive and combinable in any order, except where it is
logical that such numerical ranges are construed to add up to
100%.
[0018] Semiconductor wafers may be cut from ingots using any
conventional method known in the art. In general, ingots are cut
using conventional wire saw apparatus. An example of such a wire
saw apparatus is disclosed and illustrated in U.S. Pat. No.
6,568,384. The wire used in the saw is such that abrasive particles
such as carborundum, diamond, silicon carbide or other abrasive
particles having a size of #100 to #6000 adhere to it. A bonding
agent may be applied to the wire to cause the abrasives to adhere
to it.
[0019] Slurries used in the cutting process are a mixture of free
abrasive particles and an oil base or aqueous solution base. The
oil base used is a mineral oil in which a dispersant and thickener
have been mixed. The aqueous solution base used includes various
additives added to 30% to 80% water or polyethylene glycol. Silicon
carbide, green silicon carbide or various metal and oxide grits may
be used as the free abrasive particles. Typically silicon carbide
having particles sizes of #600 to #1000 is used.
[0020] The specific formulations and amounts of components in
slurries may vary as determined by preferences of workers.
Typically, the slurry is a mixture of a solution of mineral oil as
the base to which silicon carbide is added. The mixture ratio may
range from base:silicon carbide=1:1 to 1:1.3. An aqueous base
slurry may include silicon carbide added to 30% to 80% water or
polyethylene glycol in a mixture of base:silicon carbide of 0.8 to
0.9:1.
[0021] The process of cutting the ingot to form a semiconductor
wafer results in many contaminants on the surface of the wafer.
Metals from the wire saw such as iron, copper, lead and zinc as
well as the organic materials, such as mineral oil and polyethylene
glycol and abrasive particles as well as other additives which are
conventionally included in slurries coat the surfaces of the wafer.
Immediately prior to any further processing steps the semiconductor
wafer is cleaned using an aqueous composition including one or more
acids chosen from polycarboxylic acids and inorganic acids.
Immediately following the application of the aqueous acid
composition the semiconductor wafer is then cleaned with an aqueous
alkaline solution. In the first step of cleaning the semiconductor
wafer using the aqueous composition including the one or more acids
chosen from polycarboxylic acids and inorganic acids, substantially
all of the metal contaminants are removed from the wafer. In the
second step of cleaning the semiconductor wafer using the aqueous
alkaline solution, substantially all of the organic materials and
abrasives are removed from the wafer. In addition to removing
organic materials and abrasives, the aqueous alkaline composition
may simultaneously micro-etch the wafer surface. Optionally, a
third cleaning step or second aqueous acid composition may be
applied to the wafer immediately following the alkaline cleaning
step to remove any remaining contaminants.
[0022] The cleaning compositions may be applied to the
semiconductor wafer by any suitable method known in the art. The
semiconductor wafer may be immersed in the cleaning compositions,
the cleaning compositions may be sprayed on the semiconductor wafer
or the cleaning compositions may be used in conventional ultrasonic
cleaning processes. The cleaning compositions may be applied at
temperature ranges from at least room temperature and higher,
typically from room temperature to 100.degree. C. Optionally, the
semiconductor wafer may be rinsed with deionized water. After the
semiconductor wafer is cleaned of contaminants, it is processed
using conventional methods for use in manufacture of solar
cells.
[0023] The cleaning method is suitable to clean semiconductor
wafers in general and wafers used in the manufacture of solar
cells. Semiconductor wafers may be crystalline or amorphous. The
crystalline type may be monocrystalline or polycrystalline. In the
manufacture of solar cells, the cleaning method is used prior to
texturing a surface of a semiconductor. Numerous methods and
compositions for texturing are well known in the solar cell
industry. Such texturing improves the reflectance of incident light
on the wafer surface and improves the overall efficiency of the
solar cell. A clean surface is important in achieving optimum
appearance and efficiency of solar cells. Any contaminants on the
surface of the wafer may ultimately compromise the performance of
the solar cell. Accordingly, both the acid and alkaline cleaning
compositions do not include any components which interfere with the
performance of the wafers in solar cells and which may provide a
source of metal contamination. As mentioned above, the method
removes substantially all of the metals from the wafer surface.
Iron and copper are especially difficult to remove using water.
Iron in particular is problematic because it forms iron oxide on
the wafer surface. Once iron oxide forms on the wafer surface it is
very difficult to remove and compromises the texturing process and
the overall incident light absorbance of the wafer. Further, if
substantial amounts of iron remain on the wafer surface during
texturing, the black spots characteristic of iron oxide formation
become worse. The first step of the cleaning method using the
aqueous acid composition including one or more acids chosen from
polycarboxylic acids and inorganic acids removes substantially all
of the metal contaminants including iron, thus preventing or
reducing iron oxide formation on the wafer. Additionally, initial
cleaning of the wafer surface with the aqueous acid composition to
remove metal contaminants instead of a conventional alkaline
process eliminates the undesired partial texturing, as indicated by
white spots on the surface of the wafer. The aqueous acid cleaning
step typically reduces the overall metal contamination to levels in
the ppb range. Typically, after the aqueous acid cleaning step is
done the metal contaminants may range from 0 ppb to 10 ppb.
[0024] The acids may be included in the aqueous acid cleaning
composition in amounts of 0.05 wt % to 20 wt %, or such as from 0.5
wt % to 15 wt %, or such as from 1 wt % to 10 wt %. Polycarboxylic
acids include, but are not limited to, aliphatic polycarboxylic
acids such as, but not limited to, dicarboxylic acids such as, but
not limited to, oxalic acid, malonic acid, succinic acid, glutaric
acid, adipic acid, pimelic acid, maleic acid, fumaric acid and
phthalic acid, tricarboxylic acids such as, but not limited to,
trimellitic acid tricarballylic acid and oxypolycarboxylic acids
such as, but not limited to, oxydicarboxylic acids such as, but not
limited to, tartaric acid, malic acid and oxytricarboxylic acids
such as, but not limited to, citric acid. Typically, dicarboxylic
acids, oxydicarboxylic acids and oxytricarboxylic acids are used in
the aqueous acid cleaning compositions. More typically,
dicarboxylic acids and oxytricarboxylic acids are used. Most
typically, oxalic acid and citric acid are used.
[0025] Inorganic acids include, but are not limited to,
hydrochloric acid, nitric acid and sulfuric acid. When an inorganic
acid is used in the cleaning compositions, typically the acid is
hydrochloric acid.
[0026] Application of the aqueous alkaline cleaning composition
immediately following application of the aqueous acid composition
removes substantially all of the organic materials, abrasives and
conventional additives used in slurries. Such contaminants also
compromise the appearance and efficiency of any solar cell where
the wafer is used. Alkaline compounds are included in the aqueous
alkaline compositions in amounts of 0.2 wt % to 30 wt %, or such as
from 0.5 wt % to 10 wt %. Alkaline compounds which are included in
the aqueous alkaline composition include, but are not limited to,
sodium carbonate, potassium carbonate, sodium bicarbonate, sodium
hydroxide, potassium hydroxide, sodium metasilicate,
triethanolamine, tetra-methyl ammonium hydroxide (TMAH) and
mixtures thereof. Typically, the alkaline compounds used are sodium
carbonate or sodium hydroxide or mixtures thereof. When sodium
hydroxide is included in the aqueous alkaline cleaner, in addition
to removing contaminants, it also micro-etches the wafer surface.
Micro-etching means that the surface roughness of the semiconductor
wafer does not exceed 1 .mu.m (peak-to-valley height).
[0027] In addition to one or more acids in the aqueous acid
composition and one or more alkaline compounds in the aqueous base
composition, one or more surfactants may be added to the cleaning
compositions. Such surfactants may be nonionic, anionic, cationic,
amphoteric and gemini (dimeric) surfactants. Typically, the
surfactants used in the cleaning compositions are nonionic. The
surfactants assist in removing organic materials and abrasive
particles as well as metals which may have been missed by the
aqueous acid cleaning composition. Surfactants are included in the
cleaning compositions in amounts of 0.001 wt % to 1 wt %, or such
as from 0.0025 wt % to 0.5 wt %.
[0028] One type of surfactant which may be used includes, but is
not limited to, polyols. Such polyols include, but are not limited
to, diols (e.g. ethylene glycol, propylene glycol, etc.), triols
(e.g. glycerol, butanetriol, etc.), tetraols (e.g. erythritol,
threitol, pentaerythritol, etc.), pentaols (e.g. xylitol, arabitol,
ribitol, etc.), hexaols (e.g. sorbitols, mannitol, galactitol,
etc.), aldo- or keto-triose, -tetrose, -pentose, -hexose, -heptose,
etc. (e.g. glyceraldehyde, erythrose, threose, ribose, arabinose,
fructose, sorbose, glucose, galactose, mannose), di-, tri-, oligo-
or polysaccharides, such as sucrose, cellobiose, isomaltose,
maltose, maltotriose, starch, cellulose, hemicellulose, and the
like. Additionally, non-reducing sugars derived from mono-, di-,
tri-, oligo- or polysaccharides, e.g. alkyl glycosides like
methyl-glycoside, other sugars such as trehalose, isotrehalose,
raffinose, stachyose, etc.) may also be used. Other polyols include
aldonic acid, aldonic acid salts (e.g. methyl gluconate, sodium
gluconate, etc.), aldonic lactones (e.g. gluconolactone, etc.),
aldaric acid esters or salts (e.g. dimethyltartarate, diamonium
tartarate, etc.). Polyols that are useful also include polyols
which have been reacted with ethylene oxide or other alkylene
oxides to provide alkoxylated polyols. Also included are ethers and
esters of polyols.
[0029] Polyols such as glycoside and polyglycoside compounds
including alkoxylated glycosides may be used. A useful
polyglycoside is one according to the formula:
R.sub.1O--(C.sub.pH.sub.2pO).sub.r--(Z).sub.x (I)
where Z is derived from glucose, R.sub.1 is a hydrophobic group
selected from alkyl groups, alkylphenyl groups, hydroxyalkylphenyl
groups as well as mixtures thereof, wherein the alkyl groups may be
straight chained or branched, which contain from 8 to 18 carbon
atoms, p is 2 or 3, r is an integer from 0 to 10, and x is a value
from 1 to 8.
[0030] A further alkyl glycoside is represented by the formula
below:
R.sub.2O--(R.sub.3O).sub.y-(G).sub.xZ.sub.b (II)
where R.sub.2 is a monovalent organic radical containing from 6 to
30, preferably from 8 to 18 carbon atoms; R.sub.3 is a divalent
hydrocarbon radical containing from 2 to 4 carbon atoms; 0 is an
oxygen atom; y is an integer from 0 to 1; G is a moiety derived
from a reducing saccharide containing 5 or 6 carbon atoms; and x is
an integer from 1 to 5; Z is O.sub.2M.sup.1,
##STR00001##
O(CH.sub.2), CO.sub.2M.sup.1, OSO.sub.3M.sup.1, or
O(CH.sub.2)SO.sub.3M.sup.1; R.sub.4 is (CH.sub.2)CO.sub.2M.sup.1 or
CH.dbd.CHCO.sub.2M.sup.1; (with the proviso that Z can be
O.sub.2M.sup.1 only if Z is in place of a primary hydroxyl group in
which the primary hydroxyl-bearing carbon atom, --CH.sub.2OH, is
oxidized to form a
##STR00002##
group); b is a number of from 0 to 3k+1 preferably an average of
from 0.5 to 2 per glycosal group; k is an integer of from 1 to 10;
M.sup.1 is H.sup.+ or an organic or inorganic counter-ion,
particularly cations such as, for example, an alkali metal cation,
ammonium cation, monoethanolamine cation, or calcium cation.
[0031] Examples of such alkylglycosides as described above include
are APG.TM. 325 CS Glycoside.RTM. which is described as being a 50%
C.sub.9-C.sub.11 alkyl polyglycoside, also commonly referred to as
D-glucopyranoside, (commercially available from Henkel Corp, Ambler
Pa.) and Glucopon.TM. 625 CS which is described as being a 50%
C.sub.10-C.sub.16 alkyl polyglycoside, also commonly referred to as
a D-glucopyranoside, (available from Henkel Corp., Ambler Pa.).
[0032] Exemplary alkyl glycosides suitable for use in the practice
of this invention include those represented by the formula:
##STR00003##
where R.sub.5 is an alkyl group, preferably a linear alkyl chain,
such as C.sub.8 to C.sub.16 alkyl groups; q is an integer value of
from 0-3, inclusive.
[0033] Examples of such alkylpolyglycoside compounds according to
structure V include: where R is C.sub.8 to C.sub.10 alkyl chains
(Glucopon.RTM. 220 UP, Glucopon.RTM. 225 DK); where R is C.sub.8,
C.sub.10, C.sub.12, C.sub.14 and C.sub.16 alkyl chains
(Glucopon.RTM. 425); where R is C.sub.12, C.sub.14 and C.sub.16
alkyl chains (Glucopon.RTM. 600 UP, Glucopon.RTM. 625 CSUP, and
Glucopon.RTM. 625 FE, all of which are available from Henkel Corp.,
Ambler Pa.). Also useful as the alkylpolyglycoside compound is
Triton.RTM. CG-110 (Union Carbide Corp.).
[0034] Another useful alkylglycoside is GLUCOPON 325N which is
described as being a C.sub.9-C.sub.11 alkyl polyglycoside, also
commonly referred to as D-glucopyranoside (from Henkel Corp, Ambler
Pa.).
[0035] Other suitable alkyl polyglycosides that are commercially
available include, but are not limited to, GLUCOPON.RTM., or
PLANTAREN.RTM. surfactants from Henkel Corporation, Ambler, Pa.,
19002. Examples of such surfactants include but are not limited to:
[0036] 1. GLUCOPON.RTM. 225 Surfactant--an alkyl polyglycoside in
which the alkyl group contains 8 to 10 carbon atoms and having an
average degree of polymerization of 1.7. [0037] 2. GLUCOPON.RTM.
425 Surfactant--an alkyl polyglycoside in which the alkyl group
contains 8 to 16 carbon atoms and having an average degree of
polymerization of 1.6. [0038] 3. GLUCOPON.RTM. 625 Surfactant--an
alkyl polyglycoside in which the alkyl groups contains 12 to 16
carbon atoms and having an average degree of polymerization of 1.6.
[0039] 4. APG.RTM. 325 Surfactant--an alkyl polyglycoside in which
the alkyl groups contains 9 to 11 carbon atoms and having an
average degree of polymerization of 1.6. [0040] 5. GLUCOPON.RTM.
600 Surfactant--an alkyl polyglycoside in which the alkyl groups
contains 12 to 16 carbon atoms and having an average degree of
polymerization of 1.4. [0041] 6. PLANTAREN.RTM. 2000 Surfactant--a
C.sub.8-16 alkyl polyglycoside in which the alkyl group contains 8
to 16 carbon atoms and having an average degree of polymerization
of 1.4. [0042] 7. PLANTAREN.RTM. 1300 Surfactant a C.sub.12-16
alkyl polyglycoside in which the alkyl groups contains 12 to 16
carbon atoms and having an average degree of polymerization of
1.6.
[0043] Other suitable polyols include, but are not limited to,
alcohol alkoxylates, fatty alcohol alkoxylates, fatty sorbitan
esters and their alkoxylates, and the like. Examples of two
commercially available surfactants are the ethylene oxide and
propylene oxide surfactants Pluronic.RTM. and Tetronic.RTM.
(obtainable from BASF).
[0044] Additional surfactants include, but are not limited to,
surfactants having the following general formula:
##STR00004##
where R' is a (C.sub.1 to C.sub.6) alky or (C.sub.6 to C.sub.14)
aryl group, G is a carboxyl, sulfonyl, or phosphonyl, M is a
charge-balancing cation such as sodium, potassium, or ammonium, and
u is an integer of from 1 to 200, preferably an integer of from 2
to 200. When u is an integer of 2 or greater, G.sub.1 is the same
or different. An example of such surfactants is Newkalgen.RTM. TX-C
(obtainable from Takemoto Oil and Fat Co.) which is a phenolic
sulfonyl salt.
[0045] Another class of surfactants which may be included in the
cleaning compositions is the gemini surfactant. A gemini surfactant
(GS) includes two conventional surfactant molecules chemically
bonded together by a spacer. Two terminal hydrocarbon tails may be
short or long; two polar head groups may be cationic, anionic or
nonionic. The spacer may be short or long, flexible or rigid. The
GS need not be symmetrically disposed about the center of the
spacer. The hydrophilic and hydrophobic groups of each surfactant
moiety may be any of those known to be used in conventional
surfactants having one hydrophilic group and one hydrophobic group.
For example, a typical nonionic gemini surfactant, e.g., a
bis-polyoxyethylene alkyl ether, may contain two polyoxyethylene
alkyl ether moieties. One type of gemini surfactant which may be
included in the cleaning compositions are the nonionic gemini
surfactants disclosed in U.S. Pat. No. 5,945,393.
[0046] Optional antifoam agents also may be used in the cleaning
compositions. Such antifoam agents include, but are not limited to,
silicon compounds such as siloxanes with glycols, acetylenics and
block alkoxy copolymers. Other conventional antifoam agents may be
used depending on the specific type of surfactant or combination of
surfactants in the cleaning compositions. Workers may use routine
experimentation to determine which antifoam agents are suitable for
a particular surfactant or combination of surfactants.
[0047] Although both the aqueous acid and aqueous alkaline cleaning
compositions may include one or more optional antifoam agents, the
aqueous acid cleaning compositions consist essentially of one or
more polycarboxylic acid, one or more surfactants and water; and
the aqueous alkaline cleaning compositions consist essentially of
one or more alkaline compound, one or more surfactants and water.
Typically, the surfactant is a nonionic surfactant such as a
polyol, such as an alkyl polyglycoside or a nonionic gemini
surfactant.
[0048] Optionally, after the aqueous alkaline cleaning step the
wafer may be cleaned with a second acid cleaning step. The optional
acid cleaning step is a repeat of the first acid cleaning step. The
optional acid cleaning step removes any remaining metals which were
not removed in the first acid cleaning step and the alkaline
cleaning step.
[0049] The following Examples are intended to further illustrate
the invention but are not intended to limit the scope of the
invention.
EXAMPLE 1
[0050] A monocrystalline silicon ingot was bonded to a contact
plate and an attachment jig. This was loaded on a conventional wire
saw apparatus. The wire saw was coated with abrasive silicon
carbide particles. During cutting a slurry containing polyethylene
glycol and #600 to #1000 size silicon carbide particles in a weight
ratio of 1:1 was sprayed from slurry nozzles of the apparatus onto
the ingot. The monocrystalline silicon wafers which were cut from
the ingot were then cleaned for 10 minutes at room temperature with
a 5 wt % aqueous alkaline composition made from a concentrate
composed of 0.05 wt % Glucopon.TM. 425N, 6.25 wt % sodium
carbonate, 17.5 wt % sodium hydroxide and 76.2 wt % water. Cleaning
was done by spraying the wafers with the aqueous alkaline
composition using a conventional spraying apparatus.
[0051] After cleaning, the silicon wafers were analyzed for metal
contaminants from the cutting process by abstraction with 10%
nitric acid mixture at room temperature for 2 minutes and then
conventional atomic absorption spectroscopy using a Varian Graphite
Furnace AA 2807 Zeeman Atomic Absorption Spectrophotometer. The
metal contaminants measured were copper at 1.26 ppb, iron at 1.53
ppb, lead at 0.32 ppb and zinc at 1.77 ppb.
EXAMPLE 2
[0052] A monocrystalline silicon ingot was bonded to a contact
plate and an attachment jig. This was loaded on a conventional wire
saw apparatus. The wire saw was coated with abrasive silicon
carbide particles. During cutting a slurry containing polyethylene
glycol and #600 to #1000 size silicon carbide particles in a weight
ratio of 1:1 was sprayed from slurry nozzles of the apparatus onto
the ingot. The monocrystalline silicon wafers which were cut from
the ingot were then cleaned for 5 minutes at room temperature with
a 5 wt % aqueous acid composition made from a concentrate composed
of 0.025 wt % Glucopon.TM. 425N, 10 wt % oxalic acid and the
balance water. The aqueous acid composition was applied to the
wafers by a conventional spraying apparatus. Immediately after
cleaning with the aqueous acid composition the wafers were spray
cleaned with the aqueous alkaline composition used in Example 1.
Cleaning with the aqueous alkaline composition was done for 10
minutes.
[0053] After cleaning, the silicon wafers were analyzed for metal
contaminants by the same procedure as in Example 1. The metal
contaminants measured were copper at 0.22 ppb, iron at 0.6 ppb,
lead at 0 ppb and zinc at 0.53 ppb. The aqueous acid composition
followed by application of the aqueous alkaline composition removed
significantly more metal residues than the aqueous alkaline
composition in Example 1.
EXAMPLE 3
[0054] A monocrystalline silicon ingot was bonded to a contact
plate and an attachment jig. This was loaded on a conventional wire
saw apparatus. The wire saw was coated with abrasive silicon
carbide particles. During cutting a slurry containing polyethylene
glycol and #600 to #1000 size silicon carbide particles in a weight
ratio of 1:1 was sprayed from slurry nozzles of the apparatus onto
the ingot. The monocrystalline silicon wafers which were cut from
the ingot were then cleaned for 5 minutes at room temperature with
a 5 wt % aqueous acid composition made from a concentrate composed
of 0.025 wt % Glucopon.TM. 425N, 10 wt % oxalic acid and the
balance water. The aqueous acid composition was applied to the
wafers by a conventional spraying apparatus. Immediately after
cleaning with the aqueous acid composition the wafers were spray
cleaned with the aqueous alkaline composition used in Example 1.
Cleaning with the aqueous alkaline composition was done for 10
minutes. Immediately after cleaning with the aqueous alkaline
composition the wafers were cleaned again for 5 minutes using the
aqueous acid composition.
[0055] After cleaning, the silicon wafers were analyzed for metal
contaminants from the cutting process as in Example 1. The metal
contaminants measured were copper at 0.2 ppb, iron at 0.3 ppb, lead
at 0 ppb and zinc at 0.38 ppb. The aqueous acid composition
followed by application of the aqueous alkaline composition and
then followed by cleaning with the acid cleaner a second time
further reduced metal residues.
EXAMPLE 4
[0056] A polycrystalline silicon ingot is bonded to a contact plate
and an attachment jig. This is loaded on a conventional wire saw
apparatus. The wire saw is coated with diamond abrasive particles.
During cutting a slurry containing mineral oil and #600 to 1000#
size silicon carbide particles in a weight ratio of 1:1 is sprayed
from slurry nozzles of the apparatus onto the ingot. The
polycrystalline silicon wafers which are cut from the ingot are
then cleaned for 5 minutes at room temperature with a 5 wt %
aqueous acid composition made from a concentrate composed of 0.025
wt % Glucopon.TM. 625, 10 wt % citric acid and the balance water.
The aqueous acid composition is applied to the wafers by a
conventional spraying apparatus at 50.degree. C. Immediately after
cleaning with the aqueous acid composition the wafers are spray
cleaned with a 5 wt % aqueous alkaline composition made from a
concentrate containing 0.05 wt % Glucopon.TM. 625, 6 wt % sodium
carbonate with the balance water. Cleaning with the aqueous
alkaline composition is done for 10 minutes.
[0057] After cleaning, the silicon wafers are analyzed as in
Example 1 for metal contaminants from the cutting process. The
metal contaminants such as copper, iron, lead and zinc are each
expected to be in amounts of less than 1 ppb.
* * * * *